They charged me, then two wands of fire, and I believe two potions of fiery burning, then focused on the king, umm.... arrows of frost/fire/acid, corrosive hammer +1, magic missiles from edwin. Got slowed and lightning bolted to heck by the traps. By the time the king died, there were only about 4-5 pieces left, including the queen.
I didn't know the chess battle had even been modified, I was getting my butt kicked too badly to notice I suppose.
Not sure if it's helpful any, but like I said, it seemed to happen as soon as I killed the king in the chess game, maybe after I looted him, not sure. But I had killed most of the other monsters (pawns) not the queen. The remaining ones go up in a puff of smoke, and I start lagging like heck.
Not that bad, but I've played BG1/2 all the way through multiple times without any lagging...
Pentium 4 2.8ghz, gig and a half of ram (one stick fried a while back), radeon 9800 pro, watercooled and overclocked to hell (stable....).
I teleported around and killed all of the crawlers (using near infinity for their locations...) that didn't do a lick of good....